In an electronic device factory, ultrapure water is used as flushing water. The ultrapure water is generally manufactured by processes including activated carbon treatment and RO membrane separation treatment of a later step using, as raw water, industrial water or drainage drained from a factory.
The purposes of the activated carbon treatment are to remove an oxidizer in the raw water or to remove organic matter and chromaticity, etc.
Since organic matter is absorbed and enriched in an activated carbon tower, the inside of the activated carbon tower becomes an environment in which microorganisms readily proliferate by using the organic matter as a nutritional source. Generally speaking, microorganisms cannot be present under the presence of an oxidizer. Consequently, the microorganisms do not exist in water flowing into activated carbon that is exposed to the oxidizer. However, the mechanism of removing the oxidizer from the activated carbon is involved with a catalytic degradation reaction on the surface of the activated carbon. The reaction proceeds at the upper part in the tower, leading to a situation where the oxidizer fails to be present in the middle and lower parts in the activated carbon tower. Thus, the inside of the activated carbon tower becomes a hotbed for microorganisms, and about 103 cells/ml to 107 cells/ml of the cells typically leak from the activated carbon tower.
The activated carbon tower is an indispensable apparatus for manufacturing ultrapure water as means for removing an oxidizer and for removing organic matter. However, as described above, the tower could be a hotbed for microorganisms. Accordingly, there has been a problem that when the concentration of the organic matter flowing into the activated carbon tower is high, the microorganisms flowing out from the activated carbon tower cause biofouling of a safety filter or RO membrane installed in a later step, thereby being clogged.
In order to achieve sterilization in the activated carbon tower, methods utilizing hot water sterilization or chlorine sterilization have been carried out.
The hot water sterilization is a method for passing hot water having a temperature of 80° C. or more through an activated carbon tower for one hour or more, thereby maintaining the hot water. However, this method is required to flow and maintain high-temperature hot water for a prolonged period, and thus cannot be said to be an industrially advantageous method.
Regarding the chlorine sterilization, Japanese Patent Publication H5-64782A has proposed a method including back washing by adding NaClO to back washing water. However, in this method, NaClO is going to be degraded at the surface of the lower layer of an activated carbon tower into which the back washing water flows, so that NaClO does not prevail in the entire portion of the activated carbon tower. Therefore, a sufficient sterilization effect cannot be achieved.
Recently, the environmental standard and water-quality standard have tended to increasingly become strict. It has been desired to highly purify even final effluent. In addition, for the purpose of dissolving water shortage, it has also been desired to develop a high-level technique for treating water so as to collect and regenerate various drainage.
RO membrane separation treatment enables impurities (e.g., ions, organic matter, microparticles) in water to be effectively removed. Thus, recently, the treatment has been used in a large number of fields. For example, when high-concentration TOC- or low-concentration TOC-containing drainage including acetone and isopropyl alcohol, which are drained from processes for manufacturing a semiconductor is collected and reused, a method (e.g., Japanese Patent Publication 2002-336886A) has been widely adopted, in which the drainage is first biologically treated to remove the TOC components and the biologically treated water is subjected to RO membrane treatment to be purified.
However, when the biologically treated drainage passes through an RO membrane separation device, biological metabolites which have been generated by degradation of organic matter by microorganisms may cause the membrane surface of the RO membrane to be occluded, which results in a decrease in flux.
When the TOC-containing drainage directly passes through the RO membrane separation device without biological treatment, a high TOC concentration at which the drainage flows into the RO membrane separation device causes an environment in which microorganisms readily proliferate in the RO membrane separation device. Here, in order to inhibit biofouling in the RO membrane separation device, a slime-controlling agent is usually added to the TOC-containing drainage.
As the slime-controlling agent, a chlorine-based oxidizer such as inexpensive sodium hypochlorite has been widely used. However, this may cause a polyamide-based RO membrane to be deteriorated. As a slime-controlling agent which does not cause the RO membrane to be deteriorated, Japanese Patent Publication 2006-263510A describes a slime-controlling agent for membrane separation, the agent including a combined chlorine agent produced from a chlorine-based oxidizer and a sulfamic acid compound, and a slime-controlling agent for membrane separation, the agent containing a chlorine-based oxidizer and a sulfamic acid compound.
In addition, drainage drained from an electronic device factory may contaminate a nonionic detergent which attaches to the membrane surface of the RO membrane separation device and possibly decreases the flux. Consequently, the RO membrane separation treatment cannot be applied.
To solve such problems, a method and apparatus (Japanese Patent Publication 2005-169372A) has been disclosed. In the method and apparatus, when high-concentration and low-concentration organic-matter-containing water drained from an electronic device factory or other various fields are treated and collected by using the RO membrane separation device, biofouling and a decrease in flux due to the attachment of organic matter onto the membrane surface in the RO membrane separation device are prevented to carry out long-term stable treatment, and, simultaneously, high-quality treated water is yielded by efficiently decreasing the TOC concentration in water. As such technology, five-fold excess by weight or more of a scale inhibitor per calcium ion in organic-matter-containing water are added to the organic-matter-containing water, and an alkaline agent is added to the organic-matter-containing water before, after, or at the same time as the addition of the scale inhibitor to adjust pH to 9.5 or more, followed by RO separation treatment.
In addition, a method and apparatus (Japanese Patent No. 3906855) is known. In the method and apparatus, together with a scale inhibitor added, drainage whose pH is adjusted to 9.5 or more is subjected to activated carbon treatment and, then, RO membrane separation treatment. By performing these treatments, growth of microorganisms in an activated carbon tower and an RO membrane separation device is inhibited to stably yield treated water. In this method, the activated carbon tower is provided so as to absorb and remove an oxidizer mixed in raw water and organic matter contained in the raw water.
A predetermined amount of a scale inhibitor is added to water to be treated (hereinafter, referred to as “RO supply water”) which is injected into an RO membrane separation device, and pH is adjusted to 9.5 or more. Then, the water passes through the RO membrane separation device. The above prevents biofouling and a decrease in the flux due to the attachment of organic matter onto the membrane surface in the RO membrane separation device to carry out long-term stable treatment, and high-quality treated water can be yielded by efficiently decreasing the TOC concentration in water.
Specifically, microorganisms cannot live in an alkaline range having a pH of 9.5 or more. In addition, a nonionic detergent that may decrease flux allows for detachment from the membrane surface in an alkaline range having a pH of 9.5 or more, thereby inhibiting attachment of this component onto the RO membrane surface.
By adding five-fold excess by weight or more of a scale inhibitor per calcium ion in RO supply water to the RO supply water, generation of scale is prevented.
There is a method in which five-fold excess by weight or more of a scale inhibitor per calcium ion in organic-matter-containing water is added to the organic-matter-containing water, and an alkaline agent is added to the organic-matter-containing water before, after, or at the same time as the addition of the scale inhibitor to adjust pH to 9.5 or more, followed by RO separation treatment. However, in this method, when a large amount of hardness components in raw water is present, addition of a scale dispersant is not sufficient for a scale-inhibiting effect. Accordingly, a cation-exchange tower or softening tower is provided to decrease the hardness load, and the pH is then required to be kept alkaline. In this case, Japanese Patent No. 3906855 describes that raw water is treated with an activated carbon tower, and is then subjected to treatment with a cation-exchange tower or softening tower, followed by treatment with an RO membrane separation device. In this treatment system, the cation-exchange tower or softening tower cannot be operated under highly alkaline conditions from a viewpoint of control of scale generation in the tower. Accordingly, the cation-exchange tower or softening tower and the activated carbon tower of the previous step should be operated under neutral conditions. As a result, the inside of the activated carbon tower and the cation-exchange tower or softening tower under the neutral conditions becomes a condition in which slime readily proliferates. This leads to a problem that biofilms detached from the tower cause the RO membrane separation device (or a safety filter of the RO membrane separation device) installed at a later step to be occluded.
In order to inhibit proliferation of slime, it is considered to add a disinfectant to raw water. However, as descried previously, a regular disinfectant such as sodium hypochlorite (NaClO) is largely removed in the activated carbon tower. Therefore, in the cation-exchange tower or softening tower following the step of the activated carbon tower, the sterilization effect cannot be achieved, and the proliferation of slime cannot be inhibited.